Silicate-Enhanced Heterogeneous Flow-Through Electro-Fenton System Using Iron Oxides under Nanoconfinement

An efficient strategy for BDD electrode drive electro-catalysis triggering active species on lincomycin and antibiotic resistance genes removal: Electron transfer based on calculation modeling

Identifying the degradation pathway and the final by-products is essential, as their ecological risks are pertinent to the advancement of this technology and its potential application in practical environmental pollution treatment. Elucidating the reaction mechanisms of the degradation system represents the most effective strategy for controlling this process. This study thoroughly revealed that indirect oxidation predominates throughout the electrochemical system, while direct oxidation serves a significant auxiliary role under the synergistic influence. It elucidates the critical importance of electron transfer behavior at the electrode surface for pollutant degradation and unveil potential mechanisms underlying primary degradation reactions via integrating charge density differences and Bader atomic charge analysis. In situ electrochemical infrared spectroscopy (In situ EC-FTIR) and density functional calculation (DFT) were used to analyze the final by-product generation path. It further elucidated the correlation between antibiotic resistance gene (ARGs) and binding strength among base pairs. The oxidative stress process of antibiotic resistance bacteria (ARB) was explained in detail. To comprehensively assess the impact of electrochemical treatment on environmental microbial communities, combined horizontal gene transfer (HGT) experiments were conducted to confirm that electrolytically treated wastewater does not induce ecological stress effects on microorganisms. Finally, a small cyclic electrochemical system was employed to evaluate both ecological impacts and economic benefits associated with wastewater treatment, thereby providing a novel theoretical framework for this domain.

Efficient antibiotic tetracycline degradation and toxicity abatement via the perovskite-type CaFexNi1-xO3 assisted heterogeneous electro-Fenton system

As one of the emerging contaminants, antibiotics are posing a great threat to the human health and environment, which requires effective treatment methods. Heterogeneous electro-Fenton is a promising technique for organic contaminant elimination, but preparation of an appropriate heterogeneous electro-Fenton catalyst still remains challenging. In this work, the feasibility of perovskite-type CaFexNi1-xO3 as heterogeneous electro-Fenton catalyst for tetracycline (TC) removal and toxicity abatement has been explored. It was found that, among the examined CaFexNi1-xO3 catalysts with different Ni doping amount, CaFe3/4Ni1/4O3 exhibited the best performance, achieving 92.1 % TC removal within 30 min without pH adjustment in the presence of 0.05 M Na2SO4 electrolyte. Choosing Cl--containing electrolyte enabled further improvement towards TC elimination. In addition, the CaFe3/4Ni1/4O3 based heterogeneous electro-Fenton system presented other advantages including good recyclability and universal applicability, and significant toxicity reduction (verified via both ECOSAR simulation and soybean germination test). The TC degradation pathways were elucidated through identification of intermediate products and DFT calculations. Mechanism investigations revealed that there existed a strong synergy between Fe and Ni, and ·OH and ·O2- played the primary roles in the system while 1O2 played an auxiliary role. This study presented a promising heterogeneous electro-Fenton catalyst for degradation of antibiotics such as tetracycline.

Enhanced oxygen diffusion and catalytic performance of self-breathing CB/CNT cathodes for high-efficiency H2O2 production (in dual-chamber reactors)

A novel self-breathing gas diffusion electrode was developed by loading carbon nanotubes (CNTs) and carbon black (CB) onto the surface of graphite felt through vacuum filtration. This electrode features a well-structured mesoporous network and a stable three-phase interface, which enable efficient oxygen mass transfer and enhance the self-breathing capability. The incorporation of carbon nanotubes and carbon black significantly boosts the electrode's catalytic performance. In a dual-chamber reactor operating at a current density of 12 mA/cm2 and an initial pH of 3, the system achieved an H2O2 concentration of 4691 mg/L within 1 h, with an energy consumption of 6.58 kWh/kg H2O2 substantially outperforming conventional gas diffusion electrodes. The dynamic pH regulation in the dual-chamber system optimizes the 2e- ORR pathway, leading to corresponding changes in proton transfer pathways and adsorbed species within the Helmholtz plane. Additionally, the presence of reactive hydrogen (H∗) enhances the chemisorption of O2 and facilitates its hydrogenation to form the ∗OOH intermediate. The electrode exhibited excellent stability, maintaining H2O2 yields above 4000 mg/L over 5 cycles and nearly complete degradation of the simulated contaminants within 30 min in an electro-Fenton system application. These results highlight the electrode's potential for efficient H2O2 synthesis and environmental remediation.

Ultrafast Water Purification by Template-Free Nanoconfined Catalysts Derived from Municipal Sludge

Nanoconfinement at the interface of heterogeneous Fenton-like catalysts offers promising avenues for advancing oxidation processes in water purification. Herein, we introduce a template-free strategy for synthesizing nanoconfined catalysts from municipal sludge (S-NCCs), specifically engineered to optimize reactive oxygen species (ROS) generation and utilization for rapid pollutant degradation. Using selective hydrofluoric acid corrosion, we create an architecture that confines atomically dispersed Fe centers within a micro-mesoporous carbon matrix in situ. This method maximizes the utilization of silicon and aluminum content from sludge, prevents metal agglomeration, and precisely regulates the chemical environment of Fe active sites. As a result, the S-NCCs promote a transition from nonradical to hybrid radical/nonradical reaction mechanisms, significantly enhancing ROS efficiency, stability, and pollutant degradation rates. These catalysts demonstrate exceptional pollutant removal performance, achieving a 261-fold increase in degradation efficiency for compounds such as phenol and sulfamethoxazole compared to unconfined analogs, outperforming most state-of-the-art Fenton-like systems. Our findings highlight the transformative potential of nanoconfined catalysis in environmental applications, providing an effective and scalable solution for sustainable water purification.

Novel millimeter-sized honeycomb-like Fe/Fe3C@HBC from waste cotton textiles towards rapid degradation of ofloxacin via activation of H2O2

Although multiphase catalysts with large sizes exhibit excellent recyclability and low toxicity in heterogeneous Fenton reactions, their reactivity, reusability and storage stability for degradation of organic contaminants still need improvement, which is essential for treating complex wastewater and ensuring environmental sustainability. In this study, the waste cotton textiles were firstly used as the carbon source to generate a novel millimeter-sized catalyst (Fe/Fe3C@HBC) with a honeycomb-like structure, which could effectively activate H2O2 to realize rapid removal of ofloxacin (OFL) (100% in 10 min). It achieved remarkable removal performance across a broad temperature range (4-40 °C) and high-concentration OFL. It even demonstrated excellent removal towards other typical contaminants (Tetracycline, Ciprofloxacin, Methylene Blue, Rhodamine B, Malachite Green), showing outstanding storage stability, physical structural stability, reusability and separation characteristics. Whereafter, its removal mechanism was also explored, showing that it was entirely dependent on the degradation by the reactive oxygen species (ROS), including •OH, O2•- and 1O2, as well as the persistent free radicals from the catalyst. Moreover, the honeycomb-like structure promoted the effective utilization of H2O2, facilitated the generation of •OH and expedited the accumulation of OFL on the catalyst surface. Fe/Fe3C (inside of the catalytic instead of in the reaction solution) was essential for the degradation. Finally, the OFL degradation pathways and toxicity predictions were also proposed. Overall, this innovation supports cleaner water resources and enhances public health, demonstrating a significant step forward in environmental remediation technologies.

Dual heteroatom-doped porous biochar from chitosan/lignosulfonate gels for enhanced removal of tetracycline by persulfate activation: Performance and mechanism

Rational design of carbon material structures is essential for enhancing the performance of persulfate-based advanced oxidation processes (PS-AOPs) in water purification. In this study, a self-doping and self-templating strategy was devised to produce N, S co-doped biochar catalysts through pre-cryocrushing and carbonization procedures employing chitosan (N-source) and lignosulfonate (S-source) derived from biomass waste. The as-synthesized materials exhibited excellent performance in removing tetracycline (TC) through a synergistic process of adsorption and catalytic activation. Mechanistic studies confirmed that electron transfer serves as the primary pathway, while singlet oxygen plays an auxiliary role. Furthermore, the toxicity of the degradation system, the impact of the complex water matrix, and the reusability of the catalysts were thoroughly investigated. Overall, this work is devoted to the treatment and application of biomass waste, which provides a feasible method for synthesizing heteroatom-doped biochar and offers valuable insights into the critical role of heteroatom-doped carbocatalysts in non-radical activation of persulfate.

Stability challenges of transition metal-modified cathodes for electro-Fenton process: A mini-review

Electro-Fenton (EF) process with transition-metal (TM) modified cathode has been regarded as a green and promising technology for wastewater treatment. Recently, breakthroughs in boosting catalyst activity for both two-electron oxygen reduction reaction (2e- ORR) and Fenton's reaction have gained intensive attention. However, achieving long-term stability of catalysts remains challenging, but is decisive for large-scale applications. This minireview provides fundamental understanding on the activity-stability correlation and the deactivation mechanisms of TM-based catalysts in EF systems, focusing on physical and chemical evolution, metal dissolution, catalyst detachment and structure collapse during long-term electrolysis. Subsequently, ongoing efforts from catalyst design to electrode engineering to stabilize the metal active sites are highlighted. Finally, the challenges and future perspectives in developing active and durable TM-modified cathodes are discussed, serving as a roadmap towards the large-scale application of EF process for wastewater treatment.

Confined iron-based nanomaterials for water decontamination: Fundamentals, applications, and challenges

Nanotechnology-enabled water treatment is the most attractive approach to realizing advanced purification of contaminated waters that challenge the efficacy of traditional water treatment technologies. Confining nanomaterials inside porous scaffolds or substrates is one of the most effective strategies to push nano-enabled water treatment technologies forward from laboratory to field application. As flourishingly reported, confinement effects induce significantly improved decontamination efficiency, such as enhanced adsorption capacity, reaction kinetics, stability, and selectivity. In this review, first we provide an overview of the general fundamentals of nanoconfinement effects and their implications in environmental remediation. Next, we review confined Fe-based nanomaterials, such as different polymorphs of iron-oxides, oxyhydroxides, zero-valent iron, and single-atom iron as representative materials towards their applications in nanoconfinement systems for water decontamination. Finally, we propose future studies based on the missing scientific fundamentals regarding nanoconfinement effects and challenges for translating unique and promising nanoconfinement observations to engineering applications of confined nanomaterials-driven water treatment technologies.

An S-Scheme MOF-on-MXene Heterostructure for Enhanced Photocatalytic Periodate Activation

Fully understanding the periodate (PI) activation system is still a great challenge, which calls for efficient heterogeneous catalysts with a sophisticated structure. Herein, we developed "MOF-on-MXene" heterostructures. By constructing S-scheme heterostructures MXene/Z67450, the internal electric field is generated via the Ti-O-Co bonds at the interface, favoring the excitation of the photogenerated electrons, providing a driving force for accelerating the charge transfer, and enhancing redox performances. Further contributed by the synergy of Ti-O-Co and Co-N4 bonds, the MXene/Z67450 composites exhibit enhanced ability for activating the periodate system to degrade organic pollutants via building a donor-catalyst-acceptor system. In the presence of periodate and light, MXene/Z67450 degraded ∼100% of tetracycline hydrochloride (TCH) in only 10 min. The active sites of the heterostructures can react with the periodate and give the intermediate MXene/Z67450-PI (*PI). As a result, it efficiently reduced the PI adsorption energy and promoted the decomposition of PI and the formation of holes/electrons, singlet oxygen (1O2) as well as hydroxyl radical (•OH). In addition, the MXene/Z67450 composites exhibit high stability, reusability, selectivity, and environmental robustness. Our study provides a research direction for rationally designing MXene-based heterojunctions and applying them in the periodate activation system.

Mechanism insights into customized 2D nanoconfined catalyst via peroxymonosulfate activation for efficient sulfamethoxazole degradation: Key roles of electronic structure and non-radical pathway

The technology to solve the problem of the efficient pollutant removal in peroxymonosulfate (PMS) activation was the ultimate goal. There was an urgent need to achieving higher catalytic activity and oxidation efficiency. Herein, we present a MgAl-based layered double hydroxide assembled as a 2D confined catalyst (MgAl-Co-LDH) with Co metal in chelated form (Co-EDTA) for highly efficient PMS activation degrading sulfamethoxazole (SMX). Co-EDTA as an active site enlarged the interlayer height of MgAl-LDH to form a nanoconfined space. The confinement interlayer structure acted as a mediator for electron transfer, which improved the effective collision of active sites with PMS and SMX. The confined catalyst had a rate constant of 0.2262 min-1, which was much superior to the non-confined catalyst by 8.76 times. A series of experiments proved that the reactive species transformed the radical pathway into singlet oxygen (1O2). The density functional theory calculations proved that the capability of PMS cleavage was optimized and modulated the electronic structure of MgAl-Co-LDH, which enhanced the reactivity of the D-band center electrons of Co-active sites. This study offered a method to investigate the catalytic degradation mechanisms of confined catalysts used in wastewater treatment.

Angstrom Confinement-Triggered Adaptive Spin State Transition of CoMn Dual Single Atoms for Efficient Singlet Oxygen Generation

To achieve high selectivity in the transformation from peroxymonosulfate to singlet oxygen, adaptive tuning of atomic spin state as the peroxymonosulfate structure varied is crucial. The angstrom confinement can effectively tune spin state, but developing an adaptive angstrom-confined atomic system is challenging. Angstrom-confined cobalt (Co) manganese (Mn) dual single atoms within flexible 2D carbon nitride interlayer are constructed to drive adaptive tuning of spin state by changing atomic coordination under angstrom confinement. The in situ characterizations and density functional theory calculations showed that medium-spin Co in Co─N4 absorbed electrons after the adsorption of peroxymonosulfate on CoMn dual single-atom sites and then cleaved O─H of peroxymonosulfate to facilitate *SO5 generation, while the introduction of *SO5 increased interlayer distance and then cleaved Co─N and Mn─N, resulting in the spin state transition from medium to high. Subsequently, the high-spin Co and Mn in Co─N2 and Mn─N2 desorbed the *O2 from *SO5, restoring the initial medium spin state. The adaptive spin state transition enhanced 38.6-fold singlet oxygen yield compared to the unconfined control. The proposed angstrom-confined diatomic strategy is applicable to serial diatomic catalysts, providing an efficient and universal design scheme for singlet oxygen-mediated selective wastewater treatment technology at the atomic level.

Incidental iron oxide nanoclusters drive confined Fenton-like detoxification of solid wastes towards sustainable resource recovery

The unique properties of nanomaterials offer vast opportunities to advance sustainable processes. Incidental nanoparticles (INPs) represent a significant part of nanomaterials, yet their potential for sustainable applications remains largely untapped. Herein, we developed a simple strategy to harness INPs to upgrade the waste-to-resource paradigm, significantly reducing the energy consumption and greenhouse gas emissions. Using the recycling of fly ash from municipal solid waste incineration (MSWI) as a proof of concept, we reveal that incidental iron oxide nanoclusters confined inside the residual carbon trigger Fenton-like catalysis by contacting H2O2 at circumneutral pH (5.0-7.0). This approach efficiently detoxifies the adsorbed dioxins under ambient conditions, which otherwise relies on energy-intensive thermal methods in the developed recovery paradigms. Collective evidence underlines that the uniform distribution of iron oxide nanoclusters within dioxin-enriched nanopores enhances the collision between the generated active oxidants and dioxins, resulting in a substantially higher detoxification efficiency than the Fe(II)-induced bulk Fenton reaction. Efficient and cost-effective detoxification of MSWI fly ash at 278‒288 K at pilot scale, combined with the satisfactory removal of adsorbed chemicals in other solid wastes unlocks the great potential of incidental nanoparticles in upgrading the process of solid waste utilization and other sustainable applications.

Turning Solid Waste into Catalysts: A Path for Environmental Solutions

Waste, often overlooked, stands out as a prime source of valuable products, meeting the demand for natural resources. In the face of environmental challenges, this study explores the crucial role of waste-derived catalysts in sustainable practices, emphasizing the transformative potential of solid waste materials. Carbon-based catalysts sourced from agricultural, municipal, and industrial waste streams can be transformed into activated carbon, biochar, and hydrochar which are extensively used adsorbents. Furthermore, the paper also highlights the potential of transition metal-based catalysts derived from spent batteries, electronic waste, and industrial byproducts, showcasing their efficacy in environmental remediation processes. Calcium-based catalysts originating from food waste, including seashells, eggshells, bones, as well as industrial and construction waste also find an extensive application in biodiesel production, providing a comprehensive overview of their promising role in sustainable and eco-friendly practices. From mitigating pollutants to recovering valuable resources, waste-derived catalysts exhibit a versatile role in addressing waste management challenges and promoting resource sustainability. By transforming waste into valuable catalysts, this study champions a paradigm shift towards a more sustainable and resource-efficient future.

Principles, challenges and prospects for electro-oxidation treatment of oilfield produced water

The oil industry is facing substantial environmental challenges, especially in managing waste streams such as Oilfield Produced Water (OPW), which represents a significant component of the industrial ecological footprint. Conventional treatment methods often fail to effectively remove dissolved oils and grease compounds, leading to operational difficulties and incomplete remediation. Electrochemical oxidation (EO) has emerged as a promising alternative due to its operational simplicity and ability to degrade pollutants directly and indirectly, which has already been applied in treating several effluents containing organic compounds. The application of EO treatment for OPW is still in an initial stage, due to the intricate nature of this matrix and scattered information about it. This study provides a technological overview of EO technology for OPW treatment, from laboratory scale to the development of large-scale prototypes, identifying design and process parameters that can potentially permit high efficiency, applicability, and commercial deployment. Research in this domain has demonstrated notable rates of removal of recalcitrant pollutants (>90%), utilizing active and non-active electrodes. Electro-generated active species, primarily from chloride, play a pivotal role in the oxidation of organic compounds. However, the highly saline conditions in OPW hinder the complete mineralization of these organics, which can be improved by using non-active anodes and lower salinity levels. The performance of electrodes greatly influences the efficiency and effectiveness of OPW treatment. Various factors must be considered when selecting the electrode material, such as its conductivity, stability, surface area, corrosion resistance, and cost. Additionally, the specific contaminants present in the OPW, and their electrochemical reactivity must be considered to ensure optimal treatment outcomes. Balancing these considerations can be challenging, but it is crucial for achieving successful OPW treatment. Active electrode materials exhibit a high affinity for chloride molecules, generating more active species than non-active materials, which exhibit more significant degradation potential due to the production of hydroxyl radicals. Regarding scale-up, key challenges include low current efficiency, the formation of by-products, electrode deactivation, and limitations in mass transfer. To address these issues, enhanced mass transfer rates and appropriate residence times can be achieved using flow-through mesh anodes and moderate current densities, which have proven to be the optimal configuration for this process.

Atomic Confinement Empowered CoZn Dual-Single-Atom Nanotubes for H2O2 Production in Sequential Dual-Cathode Electro-Fenton Process

Single-atom catalysts (SACs) are flourishing in various fields because of their 100% atomic utilization. However, their uncontrollable selectivity, poor stability and vulnerable inactivation remain critical challenges. According to theoretical predictions and experiments, a heteronuclear CoZn dual-single-atom confined in N/O-doped hollow carbon nanotube reactors (CoZnSA@CNTs) is synthesized via spatial confinement growth. CoZnSA@CNTs exhibit superior performance for H2O2 electrosynthesis over the entire pH range due to dual-confinement of atomic sites and O2 molecule. CoZnSA@CNTs is favorable for H2O2 production mainly because the synergy of adjacent atomic sites, defect-rich feature and nanotube reactor promoted O2 enrichment and enhanced H2O2 reactivity/selectivity. The H2O2 selectivity reaches ∼100% in a range of 0.2-0.65 V versus RHE and the yield achieves 7.50 M gcat -1 with CoZnSA@CNTs/carbon fiber felt, exceeding most of the reported SACs in H-type cells. The obtained H2O2 is converted directly to sodium percarbonate and sodium perborate in a safe way for H2O2 storage/transportation. The sequential dual-cathode electron-Fenton process promotes the formation of reactive oxygen species (•OH, 1O2 and •O2 -) by activating the generated H2O2, enabling accelerated degradation of various pollutants and Cr(VI) detoxification in actual wastewater. This work proposes a promising confinement strategy for catalyst design and selectivity regulation of complex reactions.

Regulating the Electronic Structure of Cu Single-Atom Catalysts toward Enhanced Electro-Fenton Degradation of Organic Contaminants via 1O2 and •OH

Heterogeneous electro-Fenton degradation with 1O2 and •OH generated from O2 reduction is cost-effective for the removal of refractory organic pollutants from wastewater. As 1O2 is more tolerant to background constituents such as salt ions and a high pH value than •OH, tuning the production of 1O2 and •OH is important for efficient electro-Fenton degradation. However, it remains a great challenge to selectively produce 1O2 and improve the species yield. Herein, the electronic structure of atomically dispersed Cu-N4 sites was regulated by doping electron-deficient B into porous hollow carbon microspheres (CuBN-HCMs), which improved *O2 adsorption and significantly enhanced 1O2 selectivity in electro-Fenton degradation. Its 1O2 yield was 2.3 times higher than that of a Cu single-atom catalyst without B doping. Meanwhile, •OH was simultaneously generated as a minor species. The CuBN-HCMs were efficient for the electro-Fenton degradation of phenol, sulfamethoxazole, and bisphenol A with a high mineralization efficiency. Its kinetic constants showed insignificant changes under various anions and a wide pH range of 1-9. More importantly, it was energy-efficient for treating actual coking wastewater with a low energy consumption of 19.0 kWh kgCOD-1. The superior performance of the CuBN-HCMs was contributed from 1O2 and •OH and its high 1O2 selectivity.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

A Critical Review of Deep Oxidation of Gaseous Volatile Organic Compounds via Aqueous Advanced Oxidation Processes

Volatile organic compounds (VOCs) are considered to be the most recalcitrant gaseous pollutants due to their high toxicity, diversity, complexity, and stability. Gas-solid catalytic oxidation methods have been intensively studied for VOC treatment while being greatly hampered by energy consumption, catalyst deactivation, and byproduct formation. Recently, aqueous advanced oxidation processes (AOPs) have attracted increasing interest for the deep oxidation of VOCs at room temperature, owing to the generation of abundant reactive oxygen species (ROS). However, current reviews mainly focus on VOC degradation performance and have not clarified the specific reaction process, degradation products, and paths of VOCs in different AOPs. This study systematically reviews recent advances in the application of aqueous AOPs for gaseous VOC removal. First, the VOC gas-liquid mass transfer and chemical oxidation processes are presented. Second, the latest research progress of VOC removal by various ROS is reviewed to study their degradation performances, pathways, and mechanisms. Finally, the current challenges and future strategies are discussed from the perspectives of synergistic oxidation of VOC mixtures, accurate oxidation, and resource utilization of target VOCs via aqueous AOPs. This perspective provides the latest information and research inspiration for the future industrial application of aqueous AOPs for VOC waste gas treatment.

Electro-Fenton degradation of pefloxacin using MOFs derived Cu, N co-doped carbon as a nanocomposite catalyst

Electro-Fenton (EF) can in-situ produce H2O2 and effectively activate H2O2 to generate powerful reactive species for the destruction of contaminants under acidic conditions, however, the production of iron-containing sludge and requirement of low working pH significantly hinder its practical application. Herein, a novel Cu, N co-doped carbon (Cu-N@C) with metal organic framework (MOF) as a precursor was constructed and adopted for the elimination of pefloxacin (PEF) in the heterogeneous electro-Fenton (HEF) process. PEF could be almost completely removed within 1 h and total organic carbon (TOC) removal efficiency was 48.57% within 6 h. Meanwhile, Cu-N@C had good repeatability and environmental adaptability, it can still maintain excellent catalytic performance after 10 cycles, and it exhibited satisfactory remediation performance in simulated water matrix. In addition, the HEF process catalyzed by Cu-N@C also showed satisfactory degradation effect on other organic pollutants including atrazine, methylene blue, and chlorotetracycline. Under the action of impressed current, the HEF system could generate H2O2 in-situ, and the active species could be generated in the redox cycle of Cu0/Cu1+/Cu2+. Electron paramagnetic resonance and quenching experiments confirmed that •OH was the dominant active species in the degradation of organic compounds. The degradation process of PEF was studied by mass spectrometry analysis of intermediate products. This study provided a simple method to prepare MOF-based electrocatalyst, which exhibits promising application potential for treatment wastewater.

Electrochemical Hydrogen Peroxide Generation and Activation Using a Dual-Cathode Flow-Through Treatment System: Enhanced Selectivity for Contaminant Removal by Electrostatic Repulsion

To oxidize trace concentrations of organic contaminants under conditions relevant to surface- and groundwater, air-diffusion cathodes were coupled to stainless-steel cathodes that convert atmospheric O2 into hydrogen peroxide (H2O2), which then was activated to produce hydroxyl radicals (·OH). By separating H2O2 generation from its activation and employing a flow-through electrode consisting of stainless-steel fibers, the two processes could be operated efficiently in a manner that overcame mass-transfer limitations for O2, H2O2, and trace organic contaminants. The flexibility resulting from separate control of the two processes made it possible to avoid both the accumulation of excess H2O2 and the energy losses that take place after H2O2 has been depleted. The decrease in treatment efficacy occurring in the presence of natural organic matter was substantially lower than that typically observed in homogeneous advanced oxidation processes. Experiments conducted with ionized and neutral compounds indicated that electrostatic repulsion prevented negatively charged ·OH scavengers from interfering with the oxidation of neutral contaminants. Energy consumption by the dual-cathode system was lower than values reported for other technologies intended for small-scale drinking water treatment systems. The coordinated operation of these two cathodes has the potential to provide a practical, inexpensive way for point-of-use drinking water treatment.

A sustainable carbon aerogel from waste paper with exceptional performance for antibiotics removal from water

In this work, a sustainable 3D carbon aerogel (AO-WPC) is prepared from waste paper (WP), and used for efficient antibiotics removal from water. The AO-WPC aerogel shows good mechanical property and can recover after 100th of 30 % compression strain. The specific surface area of AO-WPC aerogel is up to 654.58 m2/g. More importantly, this aerogel reveals proper pore size distribution, including micro sized macropores between carbon fibers and intrinsic nano scale mesopores (11.86 nm), which is conducive to remove antibiotics from water. Taking tetracycline (Tc) as an example, the maximum adsorption capacity and adsorption rate of AO-WPC for Tc are as high as 384.6 mg/g and 0.510 g/(mg‧min), respectively, which exhibits significant advantages over most of the recent absorbents, and the adsorption toward Tc reveals good resistance to various environmental factors, including pH, various ions, and dissolved organic matter (DOM). Moreover, good thermal stability enables the AO-WPC aerogel to be regenerated through simple burning, and the adsorption capacity of Tc only decreases by 10.4 % after 10 cycles. Mechanism research shows that hydrogen bonding and π-π electron-donor-acceptor (EDA) interaction play the important role in the adsorption. The excellent mechanical property and adsorption performance imply good practical prospect of the AO-WPC aerogel.

Electrochemical filtration for drinking water purification: A review on membrane materials, mechanisms and roles

Electrochemical filtration can not only enrich low concentrations of pollutants but also produce reactive oxygen species to interact with toxic pollutants with the assistance of a power supply, making it an effective strategy for drinking water purification. In addition, the application of electrochemical filtration facilitates the reduction of pretreatment procedures and the use of chemicals, which has outstanding potential for maximizing process simplicity and reducing operating costs, enabling the production of safe drinking water in smaller installations. In recent years, the research on electrochemical filtration has gradually increased, but there has been a lack of attention on its application in the removal of low concentrations of pollutants from low conductivity water. In this review, membrane substrates and electrocatalysts used to improve the performance of electrochemical membranes are briefly summarized. Meanwhile, the application prospects of emerging single-atom catalysts in electrochemical filtration are also presented. Thereafter, several electrochemical advanced oxidation processes coupled with membrane filtration are described, and the related working mechanisms and their advantages and shortcomings used in drinking water purification are illustrated. Finally, the roles of electrochemical filtration in drinking water purification are presented, and the main problems and future perspectives of electrochemical filtration in the removal of low concentration pollutants are discussed.

Efficient electro-Fenton degradation of organic pollutants via the synergistic effect of 1O2 and •OH generated on single FeN4 sites

The electro-Fenton with in situ generated 1O2 and •OH is a promising method for the degradation of micropollutants. However, its application is hindered by the lack of catalysts that can efficiently generate 1O2 and •OH from electrochemical oxygen reduction. Herein, N-doped stacked carbon nanosheets supported Fe single atoms (Fe-NSC) with FeN4 sites were designed for simultaneous generation of 1O2 and •OH to enhance electro-Fenton degradation. Due to the synergistic effect of 1O2 and •OH, a variety of contaminants (phenol, 2,4-dichlorophenol, sulfamethoxazole, atrazine and bisphenol A) were efficiently degraded with high kinetic constants of 0.037-0.071 min-1 by the electro-Fenton with Fe-NSC as cathode (-0.6 V vs Ag/AgCl, pH 6). Moreover, the superior performance for electro-Fenton degradation was well maintained in a wide pH range from 3 to 10 even with interference of various inorganic salt ions. It was found that FeN4 sites with pyridinic N coordination were responsible for its good performance for electro-Fenton degradation. Its 1O2 yield was higher than •OH yield, and the contribution of 1O2 was more significant than •OH for pollutant degradation.

Synthesis of Fe-N doped porous carbon/silicate composites regulated by minerals in coal gasification fine slag for synergistic electrocatalytic treatment of phenolic wastewater

Coal gasification fine slag (CGFS), as a difficult-to-dispose solid waste in the coal chemical industry, consists of minerals and residual carbon. Due to the aggregate structure of minerals blocking pores and encapsulating active substances, the high-value utilization of CGFS still remains a challenge. Based on the intrinsic characteristics of CGFS, this study synthesized Fe-N doped porous carbon/silicate composites (Fe-NC) by alkali activation and pyrolysis for electrocatalytic degradation of phenolic wastewater. Meanwhile, minerals were utilized to regulate the surface chemical and pore structure, turning their disadvantages into advantages, which caused a sharp increase in m-cresol mineralization. The positive effect of minerals on composite properties was investigated by characterization techniques, electrochemical analyses and density functional theory (DFT) calculations. It was found that the mesoporous structure of the mineral-regulated composites was further developed, with more carbon defects and reactive substances on its surface. Most importantly, silicate mediated iron conversion through strong interaction with H2O2, high work function gradient with electroactive iron, and excellent superoxide radical (•O2-) production capacity. It effectively improved the reversibility and kinetics of the entire electrocatalytic reaction. Within the Fe-NC311 electrocatalytic system, the m-cresol removal rate reached 99.55 ± 1.24%, surpassing most reported Fe-N-doped electrocatalysts. In addition, the adsorption and electrooxidation experiment confirmed that the synergistic effect of Fe-N doped porous carbon and silicate simultaneously promoted the capture of pollutants and the transformation of electroactive molecules, and hence effectively shortened the diffusion path of short-lived radicals, which was further supported by molecular dynamics simulation. Therefore, this research provides new insights into the problem of mineral limitations and opens an innovative approach for CGFS recycling and environmental remediation.

Oxygen-containing functional groups in Fe3O4@three-dimensional graphene nanocomposites for enhancing H2O2 production and orientation to 1O2 in electro-Fenton

In electro-Fenton (EF), development of a bifunctional electrocatalyst to realize simultaneous H2O2 generation and activation efficiently for generating reactive species remains a challenge. In particular, a nonradical-mediated EF is more favorable for actual wastewater remediation, and deserves more attention. In this study, three-dimensional graphene loaded with Fe3O4 nanoparticles (Fe3O4@3D-GNs) with abundant oxygen-containing functional groups (OFGs) was synchronously synthesized using a NaCl-template method and served as a cathode to establish a highly efficient and selective EF process for contaminant degradation. The amounts of OFGs can be effectively modulated via the pyrolysis temperature to regulate the 2e- oxygen reduction reaction activity and reactive oxygen species (ROS) production. The optimized Fe3O4@3D-GNs synthesized at 750 °C (Fe3O4@3D-GNs-750) with the highest -C-O-C and -C꞊O group ratios exhibited the maximum H2O2 and 1O2 yields during electrocatalysis, thus showing remarkable versatility for eliminating organic contaminants from surface water bodies. Experiments and theoretical calculations have demonstrated the dominant role of -C-O-C in generating H2O2 and the positive influence of -C꞊O sites on the production of 1O2. Moreover, the surface-bound Fe(II) favors the generation of surface-bound •OH, which steers a more favorable oxidative conversion of H2O2 to 1O2. Fe3O4@3D-GNs were proven to be less pH-dependent, low-energy, stable, and recyclable for practical applications in wastewater purification. This study provides an innovative strategy to engineer active sites to achieve the selective electrocatalysis for eliminating pollution and reveals a novel perspective for 1O2-generation mechanism in the Fenton reaction.

Rational design of a hydrophilic nanoarray-structured electro-Fenton membrane for antibiotics removal and fouling mitigation: An intensified catalysis process in an oxygen vacancy-mediated cathodic microreactor

Electro-Fenton membranes (EFMs) can synchronously realize organic micropollutants destruction and fouling mitigation in a single filtration process with the assistance of hydroxyl radicals (•OH). Herein, a nanoarray-structured EFM (NS-EFM) was designed by assembling Fenton reactive CoFe-LDH nanowires using a low-temperature hydrothermal method. Combined with a defect-engineering strategy, the oxygen vacancies (OVac) in the CoFe-LDH nanoarrays were tailored by manipulating the stoichiometry of cations to optimize the Fenton reactivity of NS-EFMs. The optimized NS-EFM demonstrated exceptional sulfamethoxazole (SMX) removal (99.4%) and fast degradation kinetics (0.0846 min-1), but lower energy consumption (0.22 kWh m-3 per log removal of SMX). In-depth mechanism analysis revealed that the intrinsic electronic properties of OVac endowed NS-EFM with enhanced reactivity and charge transferability at metallic active sites of CoFe-LDH, thereby intensifying •OH generation. Besides, the nanoarray-structured NS-EFM built a confined microreactor space, leading to expedited •OH microflow to SMX. Meanwhile, the hydrophilic nature of CoFe-LDH nanoarrays synergistically contributed to the high flux recovery (95.0%) and minimal irreversible membrane fouling (5.0%), effectively alleviating membrane fouling within pores and on surfaces. This study offers insights into the potential of defect engineering as a foundational strategy in the design of EFMs, significantly advancing the treatment of organic pollutants and control of membrane fouling.

Sustained oxidation of Tea-Fe(III)/H2O2 simultaneously achieves sludge reduction and carbamazepine removal: The crucial role of EPS regulation

Establishing an economic and sustained Fenton oxidation system to enhance sludge dewaterability and carbamazepine (CBZ) removal rate is a crucial path to simultaneously achieve sludge reduction and harmless. Leveraging the principles akin to "tea making", we harnessed tea waste to continually release tea polyphenols (TP), thus effectively maintaining high level of oxidation efficiency through the sustained Fenton reaction. The results illustrated that the incorporation of tea waste yielded more favorable outcomes in terms of water content reduction and CBZ removal compared to direct TP addition within the Fe(III)/hydrogen peroxide (H2O2) system. Concomitantly, this process mainly generated hydroxyl radical (•OH) via three oxidation pathways, effectively altering the properties of extracellular polymeric substances (EPS) and promoting the degradation of CBZ from the sludge mixture. The interval addition of Fe(III) and H2O2 heightened extracellular oxidation efficacy, promoting the desorption and removal of CBZ. The degradation of EPS prompted the transformation of bound water to free water, while the formation of larger channels drove the discharge of water. This work achieved the concept of treating waste with waste through using tea waste to treat sludge, meanwhile, can provide ideas for subsequent sludge harmless disposal.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Sustainable and Rapid Water Purification at the Confined Hydrogel Interface

Emerging organic contaminants in water matrices have challenged ecosystems and human health safety. Persulfate-based advanced oxidation processes (PS-AOPs) have attracted much attention as they address potential water purification challenges. However, overcoming the mass transfer constraint and the catalyst's inherent site agglomeration in the heterogeneous system remains urgent. Herein, the abundant metal-anchored loading (≈6-8 g m-2) of alginate hydrogel membranes coupled with cross-flow mode as an efficient strategy for water purification applications is proposed. The organic flux of the confined hydrogel interfaces sharply enlarges with the reduction of the thickness of the boundary layer via the pressure field. The normalized property of the system displays a remarkable organic (sulfonamides) elimination rate of 4.87 × 104 mg min-1 mol-1. Furthermore, due to the fast reaction time (<1 min), cross-flow mode only reaches a meager energy cost (≈2.21 Wh m-3) under the pressure drive field. It is anticipated that this finding provides insight into the novel design with ultrafast organic removal performance and low techno-economic cost (i.e., energy operation cost, material, and reagent cost) for the field of water purification under various PS-AOPs challenging scenarios.

Additive manufacturing-derived free-standing 3D pyrolytic carbon electrodes for sustainable microbial electrochemical production of H2O2

Producing H2O2 via microbial electrosynthesis is a cost-effective and environmentally favorable alternative to the costly and environmentally hazardous anthraquinone method. However, most studies have relied on carbon electrodes with two-dimensional (2D) surfaces (e.g., graphite), which have limited surface area and active sites, resulting in suboptimal H2O2 production. In this study, we demonstrate the enhanced efficiency of microbial H2O2 synthesis using three-dimensional (3D) electrodes produced through additive manufacturing technology due to their larger surface area than conventional carbon electrodes with 2D surfaces. This work innovatively combines 3D printed pyrolytic carbon (3D PyrC) electrodes with highly defined outer geometry and internal mesh structures derived from additive manufacturing with high-temperature resin precursors followed by pyrolysis with microbial electrochemical platform technology to achieve efficient H2O2 synthesis. The 3D PyrC electrode produced a maximum of 129.2 mg L-1 of H2O2 in 12 h, which was 2.3-6.9 times greater than conventional electrodes (e.g., graphite and carbon felt). Furthermore, the scalability, reusability and mechanical properties of the 3D PyrC electrode were exemplary, showcasing its practical viability for large-scale applications. Beyond H2O2 synthesis, the study explored the application of the 3D PyrC electrode in the bio-electro-Fenton process, demonstrating its efficacy as a tertiary treatment technology for the removal of micropollutants. This dual functionality underscores the versatility of the 3D PyrC electrode in addressing both the synthesis of valuable chemicals and environmental remediation. This study shows a novel electrode design for efficient, sustainable synthesis of H2O2 and subsequent environmental remediation.

Carbon redirection via tunable Fenton-like reactions under nanoconfinement toward sustainable water treatment

The ongoing pattern shift in water treatment from pollution control to energy recovery challenges the energy-intensive chemical oxidation processes that have been developed for over a century. Redirecting the pathways of carbon evolution from molecular fragmentation to polymerization is critical for energy harvesting during chemical oxidation, yet the regulation means remain to be exploited. Herein, by confining the widely-studied oxidation system-Mn3O4 catalytic activation of peroxymonosulfate-inside amorphous carbon nanotubes (ACNTs), we demonstrate that the pathways of contaminant conversion can be readily modulated by spatial nanoconfinement. Reducing the pore size of ACNTs from 120 to 20 nm monotonously improves the pathway selectivity toward oligomers, with the yield one order of magnitude higher under 20-nm nanoconfinement than in bulk. The interactions of Mn3O4 with ACNTs, reactant enrichment, and pH lowering under nanoconfinement are evidenced to collectively account for the enhanced selectivity toward polymerization. This work provides an adaptive paradigm for carbon redirection in a variety of catalytic oxidation processes toward energy harvesting and sustainable water purification.

Confining Bismuth-Halide Perovskite in Mesochannels of Silica Nanomembranes for Exceptional Photocatalytic Abatement of Air Pollutants

Spatially confined photocatalysis has emerged as a viable strategy for the intensification of various redox reactions, but the influence of confined structure on reaction behavior is always overlooked in gas-solid reactions. Herein, we report a nanomembrane with confining Cs3 Bi2 Br9 nanocrystals inside vertical channels of porous insulated silica thin sheets (CBB@SBA(⊥)) for photocatalytic nitric oxide (NO) abatement. The ordered one-dimensional (1D) pore channels with mere 70 nm channel length provide a highly accessible confined space for catalytic reactions. A record-breaking NO conversion efficiency of 98.2 % under a weight hourly space velocity (WHSV) of 3.0×106  mL g-1  h-1 , as well as exceptionally high stability over 14 h and durability over a wide humidity range (RH=15-90 %) was realized over SBA(⊥) confined Cs3 Bi2 Br9 , well beyond its nonconfined analogue and the Cs3 Bi2 Br9 confine in Santa Barbara Amorphous (SBA-15). Mechanism studies suggested that the insulated pore channels of SBA(⊥) in CBB@SBA(⊥) endow concentrated electron field and enhanced mass transfer that render high exposure of reactive species and lower reaction barrier needs for ⋅O2 - formation and NO oxidation, as well as prevents structural degradation of Cs3 Bi2 Br9 . This work expands an innovative strategy for designing efficient photocatalysts for air pollution remediation.

Oxygen-doped carbon nanotubes with dual active cites to enhance •OH formation through three electron oxygen reduction

The electro-Fenton (EF) process generates H2O2 through the 2e- oxygen reduction reaction (ORR), which is subsequently activated to •OH by iron-based catalysts. To alleviate the potential risk of external Fe-based catalysts, along with metal dissolution in acidic or neutral environments, in this study we employed oxygen-doped carbon nanotubes (OCNT) as a bifunctional, metal-free cathode to establish a metal-free EF process for organic pollutant degradation. The results demonstrate that the metal-free electrode has excellent H2O2 accumulation (12 mg L-1 cm-1) and degrades sulfathiazole (STZ) with 97.05 % efficiency in 180 min with an explanation kinetic of 0.0189 min-1. For the first time, this enhancement came from the dual active site centers in OCNT: Ⅰ) -COOH and defects active sites were responsible for H2O2 production, Ⅱ) then -CO triggered H2O2 into •OH, avoiding the introduction of metal-based catalysts. These findings suggest that the EF system with in situ oxygen-doped cathodes have great potential for treating antibiotic wastewater.

Self-acclimation mechanism of pyrite to sulfamethoxazole concentration in terms of degradation behavior and toxicity effects caused by reactive oxygen species

Pyrite has been extensively tested for oxidizing contaminants via the activation of water molecule or dissolved oxygen, while the changing of oxidation species induced by contaminant's concentration has been largely underestimated. In this study, we revealed a self-acclimation mechanism of pyrite in terms of •OH conversion to 1O2 during the sulfamethoxazole (SMX) degradation process under oxic conditions. Two reaction stages of SMX degradation by pyrite were observed. The SMX concentration decreased by 70% rapidly in the first 12 h after the reaction was initiated, then, the removal rate began to decrease as the SMX concentration decreased. Importantly, •OH and O2•- were the dominant oxidizing species in stage one, while 1O2 was responsible for the further degradation of SMX in stage two. The self-acclimated mechanism of pyrite was proven to be caused by the conversion of oxidative species at the surface of pyrite. This process can overcome the shortages of •OH such as ultrashort lifetime and limited effective diffusion in the decontamination of micropollutant. Moreover, different reactive oxygen species will lead to different degradation pathways and environmental toxicity while degrading pollutants. This finding of oxidizing species' self-acclimation mechanism should be of concern when using pyrite for water treatment.

Structure Evolution of Iron (Hydr)oxides under Nanoconfinement and Its Implication for Water Treatment

In the development of nanoenabled technologies for large-scale water treatment, immobilizing nanosized functional materials into the confined space of suitable substrates is one of the most effective strategies. However, the intrinsic effects of nanoconfinement on the decontamination performance of nanomaterials, particularly in terms of structural modulation, are rarely unveiled. Herein, we investigate the structure evolution and decontamination performance of iron (hydr)oxide nanoparticles, a widely used material for water treatment, when confined in track-etched (TE) membranes with channel sizes varying from 200 to 20 nm. Nanoconfinement drives phase transformation from ferrihydrite to goethite, rather than to hematite occurring in bulk systems, and the increase in the nanoconfinement degree from 200 to 20 nm leads to a significant drop in the fraction of the goethite phase within the aged products (from 41% to 0%). The nanoconfinement configuration is believed to greatly slow down the phase transformation kinetics, thereby preserving the specific adsorption of ferrihydrite toward As(V) even after 20-day aging at 343 K. This study unravels the structure evolution of confined iron hydroxide nanoparticles and provides new insights into the temporospatial effects of nanoconfinement on improving the water decontamination performance.

Metal-free electrified membranes for contaminants oxidation: Synergy effect between membrane rejection and nanoconfinement

Electro-Fenton processes are frequently impeded by depletion of metal catalysts, unbalance between H2O2 generation and activation, and low concentration of reactive species (e.g., •OH) in the bulk solution. A metal-free electro-Fenton membrane was fabricated with nitrogen-doped carbon nanotube (N-CNT) and reduced graphene oxide (RGO). N-CNT acted as a catalyst for both H2O2 generation and activation, while the incorporated RGO served as the second catalyst for H2O2 generation and improved the performance of membrane rejection. The electrified membrane was optimized in terms of nitrogen precursors selection and composition of N-CNT and RGO to achieve optimal coupling between H2O2 generation and activation. The membrane fabricated with 67% mass of N-CNT with urea as the precursor achieved over 95% removal of the target contaminants in a single pass through the membrane with a water flux of 63 L m-2 h-1. This membrane also exhibited efficient transformation of various concentrations of contaminants (i.e., 1-10 mg L-1) over a broad range of pH (i.e., 3-9). Due to its good durability and low energy consumption, the metal-free electro-Fenton membrane holds promise for practical water treatment application. The concentration-catalytic oxidation model elucidated that the elevated contaminant concentration near the membrane surface enhanced the transformation rate by 40%. The nanoconfinement enhanced the transformation rate constant inside the membrane by a factor of 105 because of elevated •OH concentration inside the nanopores. Based on the prediction of this model, the configuration of the membrane reactor has been optimized.

New insights into Co3O4-carbon nanotube membrane for enhanced water purification: Regulated peroxymonosulfate activation mechanism via nanoconfinement

Co-based peroxymonosulfate (PMS) activation system with fascinating catalytic performance has become a promising technology for water purification, but it always suffers from insufficient mass transfer, less exposed active sites and toxic metal leaching. In this work, a carbon nanotube membrane confining Co3O4 inside (Co3O4-in-CNT) was prepared and was coupled with PMS activation (catalytic membrane process) for sulfamethoxazole (SMX) removal. Compared with counterpart with surface-loaded Co3O4 (Co3O4-out-CNT), the Co3O4-in-CNT catalytic membrane process exhibited enhanced SMX removal (99.5% vs. 89.1%) within residence time of 2.89 s, reduced Co leaching (20 vs. 147 μg L-1) and more interestingly, the nonradical-to-radical mechanism transformation (from 1O2 and electron transfer to SO4•- and •OH). These phenomena were ascribed to the nanoconfinement effect in CNT, which enhanced mass transfer (2.80 × 10-4 vs. 5.98 × 10-5 m s-1), accelerated Co3+/Co2+ cycling (73.4% vs. 65.0%) and showed higher adsorption energy for PMS (cleavage of O-O bond). Finally, based on the generated abundant reactive oxygen species (ROS), the seven degradation pathways of SMX were formed in system.

Atomically Dispersed Iron Regulating Electronic Structure of Iron Atom Clusters for Electrocatalytic H2 O2 Production and Biomass Upgrading

The integration of highly active single atoms (SAs) and atom clusters (ACs) into an electrocatalyst is critically important for high-efficiency two-electron oxygen reduction reaction (2e- ORR) to hydrogen peroxide (H2 O2 ). Here we report a tandem impregnation-pyrolysis-etching strategy to fabricate the oxygen-coordinated Fe SAs and ACs anchored on bacterial cellulose-derived carbon (BCC) (FeSAs/ACs-BCC). As the electrocatalyst, FeSAs/ACs-BCC exhibits superior electrocatalytic activity and selectivity toward 2e- ORR, affording an onset potential of 0.78 V (vs. RHE) and a high H2 O2 selectivity of 96.5 % in 0.1 M KOH. In a flow cell reactor, the FeSAs/ACs-BCC also achieves high-efficiency H2 O2 production with a yield rate of 12.51±0.18 mol gcat -1  h-1 and a faradaic efficiency of 89.4 %±1.3 % at 150 mA cm-2 . Additionally, the feasibility of coupling the produced H2 O2 and electro-Fenton process for the valorization of ethylene glycol was explored in detail. The theoretical calculations uncover that the oxygen-coordinated Fe SAs effectively regulate the electronic structure of Fe ACs which are the 2e- ORR active sites, resulting in the optimal binding strength of *OOH intermediate for high-efficiency H2 O2 production.

Water Flow-Driven Coupling Process of Anodic Oxygen Evolution and Cathodic Oxygen Activation for Water Decontamination and Prevention of Chlorinated Byproducts

Electrochemical advanced oxidation process (EAOP) is a promising technology for decentralized water decontamination but is subject to parasitic anodic oxygen evolution and formation of toxic chlorinated byproducts in the presence of Cl-. To address this issue, we developed a novel electrolytic process by water flow-driven coupling of anodic oxygen evolution reaction (OER) and cathodic molecular oxygen activation (MOA). When water flows from anode to cathode, O2 produced from OER is carried by water through convection, followed by being activated by atomic hydrogen (H*) on Pd cathode to produce •OH. The water flow-driven OER/MOA process enables the anode to be polarized at low potential (1.7 V vs SHE) that is lower than that of conventional EAOP whose •OH is produced from direct water oxidation (>2.3 V vs SHE). At a flow rate of 30 mL min-1, the process could achieve 94.8% removal of 2,4-dichlorophenol (2,4-DCP) and 71.5% removal of chemical oxygen demand (COD) within 45 min at an anode potential of 1.7 V vs SHE and cathode potential of -0.5 V vs SHE. To achieve the comparable 2,4-DCP removal performance, 4.3-fold higher energy consumption was needed for the conventional EAOP with titanium suboxide anode (anode potential of 2.9 V vs SHE), but current efficiency declined by 3.5 folds. Unlike conventional EAOP, chlorate and perchlorate were not detected in the OER/MOA process, because low anode potential <2.0 V vs SHE was thermodynamically unfavorable for the formation of chlorinated byproducts by anodic oxidation, indicated by theoretical calculations and experimental data. This study provides a proof-in-concept demonstration of water flow-driven OER/MOA process, representing a paradigm shift of electrochemical technology for water decontamination and prevention of chlorinated byproducts, making electrochemical water decontamination more efficient, more economic, and more sustainable.

New Insights into Selective Singlet Oxygen Production via the Typical Electroactivation of Oxygen for Water Decontamination

Selective production of singlet oxygen (1O2) as an electrophilic oxidant is crucial for the precise control of chemical targets in environmental fields. Herein, we proposed a strategy to construct a redox interface on electrodes, which can in situ produce inorganic metal hydroperoxides with appropriate oxidative ability during oxygen activation. Benefiting from atomic Cu sites (CuN4) in a copper-carbon aerogel electrode, almost complete production of 1O2 was achieved, thereby refraining the competitive formation of other reactive oxygen species. The fast electron transfer rate between CuN4 and electrogenerated H2O2 promoted the in situ formation of copper hydroperoxide (N4-Cu-OOH), thereby selectively and efficiently oxidizing intermediate O2•- to 1O2. The optimized production of 1O2 was up to 2583 μmol L-1 without additional chemical reagents. We further considered the high production of 1O2 for efficiently removing electron-rich organic pollutants from a complex water matrix. Fast kinetics was achieved and considered for removing various pollutants with electron-donating substituents in a nonradical oxidation pathway. The BPA degradation efficiency is less susceptible to the coexisting natural organic matter (NOM) and inorganic ions. Specifically, the kinetic constant for BPA removal is 34 times higher than that for a nanoparticle of a copper-carbon electrode while producing a hydroxyl radical. Our findings highlight the innovative interfacial surface engineering of an electrocatalytic O2 activation system to selectively generate 1O2 for future potential applications.

Enhanced Oxidation of Organic Compounds by the Ferrihydrite-Ferrate System: The Role of Intramolecular Electron Transfer and Intermediate Iron Species

Previous studies mostly held that the oxidation capacity of ferrate depends on the involvement of intermediate iron species (i.e., FeIV/FeV), however, the potential role of the metastable complex was disregarded in ferrate-based heterogeneous catalytic oxidation processes. Herein, we reported a complexation-mediated electron transfer mechanism in the ferrihydrite-ferrate system toward sulfamethoxazole (SMX) degradation. A synergy between intermediate FeIV/FeV oxidation and the intramolecular electron transfer step was proposed. Specifically, the conversion of phenyl methyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2) suggested that FeIV/FeV was involved in the oxidation of SMX. Moreover, based on the in situ Raman test and chronopotentiometry analysis, the formation of the metastable complex of ferrihydrite/ferrate was found, which possesses higher oxidation potential than free ferrate and could achieve the preliminary oxidation of organics via the electron transfer step. In addition, the amino group of SMX could complex with ferrate, and the resulting metastable complex of ferrihydrite/ferrate would combine further with SMX molecules, leading to intramolecular electron transfer and SMX degradation. The ferrate loss experiments suggested that ferrihydrite could accelerate the decomposition of ferrate. Finally, the effects of pH value, anions, humic acid, and actual water on the degradation of SMX by ferrihydrite-ferrate were also revealed. Overall, ferrihydrite demonstrated high catalytic capacity, good reusability, and nontoxic performance for ferrate activation. The ferrihydrite-ferrate process may be a green and promising method for organic removal in wastewater treatment.

Packed OV-SnO2-Sb bead-electrodes for enhanced electrocatalytic oxidation of micropollutants in water

Electrocatalytic oxidation is an appealing treatment option for emerging micropollutants in wastewater, however, the limited reactive surface area and short service lifetime of planar electrodes hinder their industrial applications. This study introduces an innovative electrochemical wastewater treatment technology that employs packed bead-electrodes (PBE) as a dynamic electrocatalytic filter on a dimensionally stable anode (DSA) acting as a current collector. By using PBE, the electroactive volume is expanded beyond the vicinity of the common planar anode to the thick porous media of PBE with a vast electrocatalytic surface area. This greatly enhances the efficiency of electrochemical degradation of micropollutants. The OV-SnO2-Sb PBE filter achieved a nearly 100 % degradation of moxifloxacin (MOX) in under 2 min of single-pass filtration, with a degradation rate over an order of magnitude higher than the conventional electrochemical oxidation processes. The generation of abundant radical species (•OH) and non-radical species (1O2 and O3), along with the enhanced direct oxidation, led to the outstanding performance of the charged PBE system in MOX degradation. The OV-SnO2-Sb PBE was remarkably stable, and the separation between the electroactive PBE layer and the base Ti anode allows for easy renewal of the bead-electrode materials and scaling up of the system for practical applications. Overall, our study presents a dynamic electroactive PBE that advances the electrocatalytic oxidation technology for effective control of emerging pollutants in the water environment. This technology has the potential to revolutionize electrochemical wastewater treatment and contribute to a more sustainable future environment.

Biochar-based microporous nanosheets-mediated nanoconfinement for high-efficiency reduction of Cr(VI)

Biochar-based materials have been widely used to remove Cr(VI). However, current strategies mainly focus on slow adsorption through electrostatic and functional group properties, ignoring the confinement catalytic fast kinetics caused by inherent porous properties. Herein, we designed a confinement strategy to achieve high-efficiency Cr(VI) reduction by encapsulating the catalytic reaction of Cr(VI) and oxalic acid (OA) in the micropore of PCRN-3-10-2-800. The results showed that the removal rate constant of the PCRN-3-10-2-800/OA system was 14.3 and 146.8 times higher than that of the BC-800/OA system (low porosity) and PCRN-3-10-2-800 alone (adsorption), which was highest removal rate constant in the current reported materials under the same system. The structure-activity relationship indicated that the catalytic activity of Cr(VI) depended on the micropore characteristics of the catalyst. Density functional theory calculations confirmed that nanoscale space could enhance Cr(VI) adsorption and reduce the energy barrier of the rate-determining step. The electron paramagnetic resonance spectrum demonstrated the rapid conversion of Cr(VI) to Cr(III). Furthermore, the PCRN-3-10-2-800/OA system showed good applicability and high efficiency for Cr(VI) removal (nearly 100% in 5 min) in industrial electroplating wastewater treatment. This work first proposes a nanoconfinement-induced heavy metal reduction strategy and guides biochar's universality design in wastewater treatment.

Toxicity evolution and control for the UV/H2O2 degradation of nitrogen-containing heterocyclic compounds: SDZ and PMM

This study aimed to achieve toxicity control of sulfadiazine (SDZ) and pirimiphos-methyl (PMM) via the UV/H2O2 process by optimizing the reaction parameters. The results show that both drugs had a good degradation effect under the following parameters: a H2O2 molar ratio of 1:200, and neutral conditions. SDZ and PMM could be degraded by more than 99% within 3 min, respectively. In the Daphnia magna acute toxicity assay and Vibrio fischeri inhibition assay, both SDZ and PMM exhibited a phenomenon of increasing toxicity. Additionally, through the use of density functional theory (DFT) calculation and HPLC-QTOF-MS, 21 transformation products (TPs) were identified, and the principal degradation pathways were proposed. The toxicity of the TPs was determined by comparing the QSAR prediction results with toxicity test data. As a result, under the higher UV light intensity (2300 μW/cm2) and neutral conditions, SDZ showed highest toxicity, whereas PMM showed lowest toxicity under the lowest UV light intensity (450 μW/cm2) and neutral conditions. Four main toxic TPs were identified, and their yields could be reduced by adjusting the reaction parameters. Therefore, the selection of appropriate reaction parameters could reduce the production of toxic TPs and ensure the safety of water environment.

A novel flow-through dual-system electro-Fenton for boosting PAEs removal efficiency in natural waters

In a conventional electro-Fenton system with a single cathode, it is difficult to attain both high H2O2 generation by oxygen reduction reaction (ORR) and efficient iron reduction reaction (FRR). For this study, a flow-through dual-system electro-Fenton (FT-DEF) reactor was designed to overcome this shortcoming and promote mass transfer to effectively remove dimethyl phthalate (DMP) from water. By comparing the ORR and FRR performances of four different commercial carbon electrodes, the graphite felt with the highest amount of H2O2 generation was selected as the cathode of the ORR system, and the activated carbon fiber with the best Fe (III) reduction effect was selected as another cathode of the FRR system. The ORR system and FRR system operate simultaneously to form the DEF system. The FT-DEF system displayed many advantages compared with the conventional electro-Fenton (CI-ORR), presenting an improved efficiency and low energy consumption in phthalates removal. Under optimal reaction conditions, the FT-DEF system is capable to degrade 100% DMP in 20 min, which is 25% higher than the CI-ORR, while the reaction rate constant (0.271 min-1) is 16 times that of CI-ORR system (0.017min-1). In addition, the TOC removal of FT-DEF achieving 72.3% within 2 h with energy consumption of 2.35 kW h·m-3 is much better than CI-ORR that only achieves 18.3% TOC removal within 2 h with energy consumption of 8.13 kW h·m-3. Furthermore, control parameters and mechanism of FT-DEF were investigated in detail. The main intermediate products of DMP were analyzed by UPLC-ESI-HRMS, and the possible degradation path of DMP was speculated. In addition, application of FT-DEF in three types of natural water demonstrated its universal applicability of the system.

Copper Nanowire Networks: An Effective Electrochemical Peroxymonosulfate Activator toward Nitrogenous Pollutant Abatement

Herein, we developed an electrochemical filtration system for effective and selective abatement of nitrogenous organic pollutants via peroxymonosulfate (PMS) activation. Highly conductive and porous copper nanowire (CuNW) networks were constructed to serve simultaneously as catalyst, electrode, and filtration media. In one demonstration of the CuNW network's capability, a single pass through a CuNW filter (τ < 2 s) degraded 94.8% of sulfamethoxazole (SMX) at an applied potential of -0.4 V vs SHE. The exposed {111} crystal plane of CuNW triggered atomic hydrogen (H*) generation on sites, which contributed to effective PMS reduction. Meanwhile, with the involvement of SMX, a Cu-N bond was formed by the interactions between the -NH₂ group of SMX and the Cu sites of CuNW, accompanied by the redox cycling of Cu²⁺/Cu⁺, which was facilitated by the applied potential. The different charges of the active Cu sites made it easier to withdraw electrons and promote PMS oxidation. Theoretical calculations and experimental results were combined to suggest a mechanism for pollution abatement with CuNW networks. The results showed that system efficacy for the degradation of a wide array of nitrogenous pollutants was robust across a broad range of solution pH and complex aqueous matrices. The flow-through operation of the CuNW filter outperformed conventional batch electrochemistry due to convection-enhanced mass transport. This study provides a new strategy for environmental remediation by integrating state-of-the-art material science, advanced oxidation processes, and microfiltration technology.

Minerals as catalysts of heterogeneous Electro-Fenton and derived processes for wastewater treatment: a review

Advanced oxidation processes (AOPs) such as Fenton's reagent, which generates highly reactive oxygen species, are efficient in removing biorefractory organic pollutants from wastewater. However, Fenton's reagent has drawbacks such as the generation of iron sludge, high consumption of H₂O₂, and the need for pH control. To address these issues, Electro-Fenton (EF) and heterogeneous Electro-Fenton (HEF) have been developed. HEF, which uses solid catalysts, has gained increasing attention, and this review focuses on the use of mineral catalysts in HEF and derived processes. The reviewed studies highlight the advantages of using mineral catalysts, such as efficiency, stability, affordability, and environmental friendliness. However, obstacles to overcome include the agglomeration of unsupported nanoparticles and the complex preparation techniques and poor stability of some catalyst-containing cathodes. The review also discusses the optimal pH range and dosage of the heterogeneous catalysts and compares the performance of iron sulfides versus iron oxides. Although natural minerals appear to be the best choice for effluents at pH>4, no scale-up reports have been found. The need for further development in this field and the importance of considering the environmental impact of trace toxic metals or catalytic nanoparticles in the treated water on the receiving ecosystem is emphasized. Finally, the article acknowledges the high energy consumption of HEF processes at the lab scale and calls for their performance development to achieve environmentally friendly and cost-effective results using real wastewaters on a pilot scale.

Perspective on pH adjustment in hydrometallurgical recycling of valuable metals from waste

pH adjustment was considered a simple step in the hydrometallurgy process, but its complicated operation was ignored in the past. In some industrial applications, the leachate pH was slowly adjusted by a diluted alkaline solution, with the defects of doubling the leachate volume and causing droplet hydrolysis/coagulation. Up to date, promising routes have been developed for rapid pH adjustment, especially in sealed high-temperature/pressure vessels. New routes emerged in some redox/decomposition reactions of nitrate/urea and organics. Such reactions did not start and/or were slow at room temperature but started spontaneously at high temperatures to generate/consume free H⁺. This induced pH adjustment in a rapid and homogeneous way.

Electroactive membrane with the electroactive layer beneath the separation layer to eliminate the interference of humic acid in the oxidation of antibiotics

Removing harmful antibiotics is essential to reclaiming water from municipal secondary effluent. Electroactive membranes are effective in the removal of antibiotics but challenged by the abundant coexisting macromolecular organic pollutants in municipal secondary effluent. To eliminate the interference of macromolecular organic pollutants in the removal of antibiotics, we propose a novel electroactive membrane with a top polyacrylonitrile (PAN) ultrafiltration layer and a bottom electroactive layer composed of carbon nanotubes (CNTs) and polyaniline (PANi). When filtering the mixture of tetracycline (TC, a typical antibiotic) and humic acid (HA, a typical macromolecular organic pollutant), the PAN-CNT/PANi membrane performed sequential removal. It retained HA at the PAN layer (by ∼96%) and allowed TC to reach the electroactive layer where it was electrochemically oxidized (e.g., by ∼92% at 1.5 V). The TC removal of the PAN-CNT/PANi membrane was marginally affected by HA, unlike that of the control membrane with the electroactive layer on the top that showed decreased TC removal after the addition of HA (e.g., decreased by 13.2% at 1 V). The decreased TC removal of the control membrane was attributed to the attachment (but not competitive oxidation) of HA on the electroactive layer that impaired the electrochemical reactivity. The HA removal prior to TC degradation realized by the PAN-CNT/PANi membrane avoided the attachment of HA and guaranteed TC removal on the electroactive layer. Long-term filtration for 9 h revealed the stability of the PAN-CNT/PANi membrane, and its advantageous structural design was conformed in the context of real secondary effluents.

Coupling anodic and cathodic reactions using an electrocatalytic dual-membrane system actuates ultra-efficient degradation with regulable mechanisms

The versatile reaction possibilities arising from the interaction between the anodic and cathodic reactions naturally contained in electrocatalytic membrane filtration (EMF) systems are of great valuable in meeting the current complex water treatment requirements. But currently, most studies only focus on half-cell reactions with a single electrocatalytic membrane, which limits the research progress of the EMF technology. Here we report a coupling strategy that utilizes the interaction between the anodic and cathodic reactions to actuate ultra-efficient degradation performance with regulable reaction mechanisms. An electrocatalytic dual-membrane filtration (EDMF) system was established. Six typical configurations of the EDMF system were set up and systematically investigated by adjusting the electrode distance and filtration sequence. Based on the obtained results of degradation performance and mechanisms, a regulation strategy which enabled flexible tuning of direct nonradical oxidation (e.g., h⁺) and indirect oxidation (e.g., ¹O₂, ·OH, HO₂·, O₂·^-, etc.) was proposed. In particular, cathodic reactions were found to adversely affect the anodic reactions at the relatively short electrode distance of 0.9 mm. Anodic reactions could inhibit the generation of ¹O₂ at short distance of 0.9 mm but promote its generation at long distances of 9 and 17 mm. The A-C_0.9 configuration achieved the highest degradation performance, while the C-A_9 configuration was revealed to be much more conducive to ¹O₂ production. Overall, our findings demonstrate the versatility and tunability of the reaction mechanism and performance of the EDMF system due to the flexible coupling of the anodic and cathodic reactions, which potentially lays a foundation for future development of ultra-efficient mechanism-adjustable electrocatalysis technologies.